1. Field of the Invention
The present invention relates to a light emitting diode (LED), particularly a GaN-based LED, and more particularly, to an LED having enhanced current spreading and light extraction efficiencies by incorporating a layer between a semiconductor layer and an electrode pad.
2. Discussion of the Background
An LED is a photoelectric conversion device that emits light through recombination of electrons and holes when current is applied. As an example, a GaN-based LED has an n-type semiconductor layer, an active layer, and a p-type semiconductor layer, which are sequentially arranged on a substrate. A transparent electrode layer is arranged on the p-type semiconductor layer, and a p-type electrode pad is arranged on the transparent electrode. The active layer and the p-type semiconductor layer may be partially removed so that a portion of the n-type semiconductor layer is exposed. An n-type electrode pad is arranged on an upper region of the exposed n-type semiconductor layer.
In such an LED, light is mainly emitted through a portion of the transparent electrode layer, which serves as an electrode together with the electrode pad. Therefore, when selecting a material for the transparent electrode layer, it should have strong electrical characteristics and should minimally interrupt light emission. A Ni/Au layer or an indium tin oxide (ITO) layer may be used as the transparent electrode layer. The Ni/Au layer has an excellent electrical characteristic but has a low transparency with respect to visible light. Conversely, the ITO layer has a transmittance of 90% or more with respect to visible light and thus is excellent in transparency, but has a low electrical characteristic.
In a conventional LED, an opening may be formed by etching a portion of a transparent electrode layer (particularly, an ITO layer), and a p-type electrode pad contacts the p-type semiconductor layer through the opening. In the conventional LED, a p-type tunnel layer (p++) doped with a highly concentrated p-type impurity is arranged on the p-type semiconductor layer for establishing an ohmic contact. Therefore, the p-type electrode pad contacts the p-type tunnel layer.
However, in the conventional LED, current flow is concentrated just under the p-type electrode pad, and therefore, current may not be widely spread out on the transparent electrode layer. This is because the p-type electrode pad is in direct contact with the p-type tunnel layer. The structure causes the recombination rate of electrons and holes in an active layer to decrease, thereby lowering luminous efficiency. Also, in the conventional LED, a large amount of light may be absorbed by the p-type electrode pad, and therefore lost.